Refrigerant systems typically comprise vapor-compression circuits in which a compressor circulates a refrigerant through an evaporator, an expansion device and a condenser. Typically, in a cooling system an evaporator heat exchanger is positioned within a cooled space and a condenser heat exchanger is positioned outside the space. The evaporator absorbs heat from the space whereby the refrigerant carries the heat to the condenser for discharge to the surroundings. In some systems it is desirable for the temperature within the space to be maintained within a narrow temperature band. For example, it is desirable to maintain temperatures nearly constant in refrigeration units where food products are stored.
Operation of the refrigerant system and the compressor is typically monitored by a controller, which reacts to a temperature sensed within the cooled space. Generally, the temperature within the space is regulated by controlling the flow rate of refrigerant through the vapor-compression circuit, typically by controlling operation of the compressor. Varying the refrigerant flow rate, however, changes the capacity of the vapor-compression circuit, which inhibits precise control of the temperature. For example, if the controller senses that the space is at the proper temperature, the controller can discontinue operation of the compressor. Once the temperature within the space rises above a set temperature limit, the compressor must again be activated. Such an interruption in the refrigerant system produces not only a lag in the ability of the compressor to respond the cooling demands in the space, but an undesirable interruption of the heat exchange capabilities of the condenser and evaporator in the vapor-compression circuit.
Flow of refrigerant through the vapor-compression circuit can also be controlled by placing an actively controlled valve in the vapor-compression circuit between the compressor and the evaporator. The controller issues pulsed control signals to the valve to permit intermittent bursts of refrigerant into the compressor to vary the capacity of the compressor. Thus, the compressor is not required to power down and time lags and inefficiencies in the vapor-compression circuit can be avoided. One such pulse width modulation system is described with respect to a reciprocating piston compressor in U.S. Pat. No. 6,047,556 to Lifson, which is assigned to Carrier Corporation, Syracuse, N.Y. Such a valve is, however, positioned before an intake manifold such that the capacity of the entire compressor is regulated by the valve. One other system integrates a pulse width modulation valve directly into a cylinder head of the compressor, as is described in U.S. Patent Application 2006/0218959 to Sandkoetter, which is assigned to Bitzer Kuehlmaschinenbau, Sindelfingen, Germany. Such a compressor, however, requires customized components and adds undesirable complexity to the compressor.